Xerographic material containing an inorganic photoconductor and nonpolymeric crystalline organic substances and methods of using of such material



United States ABSTRACT OF THE DISCLOSURE An electrophotographic materialis disclosed which includes a support and a photoconductive insulatinglayer coated thereon. The photoconductive insulating coating consistsessentially of an inorganic photoconductor and nonpolymeric, crystallineorganic substances. The organic substances may be acids, anhydrides,amine derivatives, phenol derivatives, esters, and aniline derivatives.Methods of using the electrophotographic material are also disclosed.

This invention relates to a xerographic printing material and method ofuse.

It is already known to produce electrophotographic printing elementsusing a photoconductive zinc oxide or other particulate photoconductorwhich is held to the surface of the support member by a film forminginsulating polymeric binding material such as a resin.

It is known that the charge holding capacity of the element depends onthe presence of the resin binder which in addition to being an insulatoralso prevents access of moisture to the photoconductor particles byvirtue of forming a continuous film around the discrete particlesembedded therein.

The normal binders which are used in the bonding of the photoconductorto the surface are insulating filmforming resins or gums, these bindersbeing polymers and as such requiring careful curing to ensure thecorrect charge holding characteristics under varying climatic conditions, and this curing must be carefully attended to in particularwhen the sheet is processed several times such as in overprinting colourwork using liquid developers, as the repeated action of the solventscontained in the liquid developers may impair the electrical propertiesof the surface by softening the resin binder if such is not sufiicientlycured to develop complete solvent resistance.

A further disadvantage of the prior art resin binder coatings is theunsatisfactory continuous tone response of such coatings caused at leastin part by the presence of the insulating resin binder, as theelectrical properties of such binder are vastly different from those ofthe particulate photoconductor and any localised concentration of suchbinder in the coating or any uneven dispersion of the photoconductorparticles in the resin matrix or any uneven wetting of thephotoconductor particles by the binder may give rise to artifacts whichform discontinuities in the necessary uniform photosensitivity of suchcoating.

It will be thus seen that the insulating resin binder coatings knownheretofore have certain disadvantages which it is the object of thisinvention to overcome.

atent C 3,406,063 Patented Oct. 15, 1968 The electrophotographic processin accordance with this instant invention comprises the steps ofproviding an electrophotographic printing element consisting of arelatively conducting backing such as paper or metal sheet havingdeposited at least on one side thereof a photoconductive layercontaining a particulate photoconductor and a nonpolymeric crystallineorganic substance, forming a pattern of electrostatic charges on thesurface of such layer, developing such pattern with electrostaticallyattractable toner material and fixing such developed pattern to the saidsurface.

The said pattern of electrostatic charges can be formed on the saidsurface of the photoconductive layer by the electrophotographic processwherein a uniform electrostatic charge is first formed on such surfacewhich is then exposed to a pattern of electromagnetic radiation wherebythe surface becomes discharged in accordance with the intensity of theincident radiation and thus a latent electrostatic image is formed onsuch surface, or alternatively the pattern of electrostatic charges canbe formed on the said surface by selective charge deposition as known inelectrostatic printing and more particularly in signal recording.

In carrying out research in connection with the improvement ofphotoconductive layers we have now surprisingly found that aphotoconductive layer can be prepared from a particulate photoconductorsuch as zinc oxide or the like and an organic nonpolymeric substancewhich is of crystalline structure and thus is not a filmformer and doesnot form a binder-like matrix containing the photoconductor particlesembedded therein and which substance thereby permits sedimentation ofthe photoconductor particles in closely packed form when applied to asurface whilst not inhibiting the charge holding and photoresponsecharacteristics of the photoconductor but forming with thephotoconductor a coating of sufiicient mechanical strength forprocessing purposes as required in the electrophotographic orelectrostatic printing process.

These substances do not require curing and therefore the production ofcoatings containing them is a much simpler matter than heretofore. It isalso found that such substances impart a more uniform charge holdingcapacity to such coatings than was possible with the resins usedheretofore, as they can be so selected that their volume resistivity isof a similar order to that of the particulate photoconductor being used.

We have found generally that as distinct from the prior art resinbinders which had to be insulators having a volume resistivity in theorder of 10 -10 ohm cm., nonpolymeric organic crystalline substanceswith volume resistivities greater than about 10 ohm cm., can be used inconjunction with particulate photoconductors. Such materials should inaddition be of nonhygroscopic nature in order that the charge holdingcapacity of the so-formed photoconductor layer should not be undulyaltered by changes in atmospheric moisture content. However suchnonpolymeric organic crystalline substances may be water soluble to theextent of forming a 10% by weight solution in water at a temperature ofthe order of 20 C., provided such solubility does not indicate thematerial to be of hygroscopic nature. In addition the nonpolymericsubstances should not react with the photoconductor at ambienttemperatures, nor should they react with the solvents used at ambienttemperatures. Thus the nonpolymeric organic crystalline substancesuseful in accordance Organic acids and anhydrides- Phthalic anhydrideZ-furoic acid Phthalic acid Salicylic acid Salicylic anhydride Maleieanhydride Maleic acid Benzoic acid Itaconic acid Adipic acidSulphosalicylic acid Alizarin Sebacic acid Iso-phthalic acid Laurie acidMyristic acid Tcrephthalic acid Amine derivativesp-Amino-azobenzinen-Tert-butylacrylamide Acetophenetidin Phenol derivativesPhenolphthalein Phenyl salicylate p-Phenyl phenol o-Phenylphenol EstersBeta-naphthyl-methyl ether l-napthyl salicylate Dimethyl terephthalateAniline derivatives Diphenyl guanidine p-Benzyl aniline An obviousadvantage of using materials according to this invention is thatrelatively thin layers such as of about 0.0015 gram per squarecentimeter or less of particulate photoconductors in closely packed formand thus of uniform photo-response can be held to a base as has not beenheretofore possible, without the presence of a resin binder whichrequires curing and introduces discontinuities.

The degree of attachment of the layer to the base employing certainorganic nonpolymeric crystalline substances as will be seen in theexamples following this disclosure, is surprisingly at least as good asthat of the prior art resin binder coatings.

However, when employing certain organic nonpolymeric crystallinesubstances particularly at relatively high pigment concentrations, itmay be found that for some purposes the attachment of the layer to thebase may not be sufiieient to allow subsequent extensive handling afterdevelopment of an image, such layer is nevertheless capable of affordingsufficient fixing to allow normal charging and exposure to an imagepattern and subsequent development, and if the final image is to befixed in the more permanent manner then a fixing medium may be usedeither in the liquid developer itself or subsequently applied, whichcould consist of such materials as hydrogenated rosin, esters ofhydrogenated rosin, the long oil alkyds, the acrylic resins, calciumresinate and similar substances, which substances can conveniently beapplied to the surface by dissolving them in a carrier liquid which willevaporate to leave the substance as a protective medium for the image.Alternatively, the surface containing the developed image may be sprayedwith a protective substance.

In the electrophotographic layers in accordance with this invention theproportion of the particulate photoconductor to the organic nonpolymericcrystalline substance can be in the range of 1-8 parts by weight of thephotoconductor to 1 part of the crystalline substance, depending on thedensity of the photoconductor and on the properties of the organicnonpolymeric substance. Mixtures of organic nonpolymeric crystallinesubstances can be also used.

The photoconductive coating material in accordance with this inventionis prepared by first dissolving the non- 2,4,6-trichloroaniline 4polymeric substance in a solvent therefor and subsequently dispersingthe particulate photoconductor in the required quantity in such solutionby ball-milling or other methods. Alternatively, the nonpolymericsubstance can be mixed with the particulate photoconductor, then thesolvent for the nonpolymeric substance can be added and such mixture canthen be homogenised. The thus prepared coating material can be thenapplied by any known method to a base material and for such purpose thecoating material can be diluted by the addition of more solvent ordiluent.

Sensitising dyes as known in the prior art can be also added to thecoating material to enhance or to affect the spectral response of thecoating.

A further advantage will be thus seen to lie in the method of preparingthe coating materials in accordance with this invention in that it isonly necessary to break up the pigment aggregates and to ensure evenparticle distribution without prolonged milling operations as arenecessary with the prior art resin binders, because in the layersprovided by this invention there is no adsorbed layer of binder materialaround the discrete pigment particles since the organic nonpolymericcrystalline substances are not film-formers and thus lack wettingproperties in the sense that this term is used in connection withconventional resinous materials.

It will be apparent that the photoconductive coatings to be used withthe liquid developing process can contain only such nonpolymericsubstances which are substantially insoluble in the carrier liquidcontained in the liquid developer.

To enable the invention to be fully appreciated some examples will noWbe given and it is to be noted that these examples refer to layers inwhich particulate photoconductors are held to a base such as paper bymeans of closely packed sedimentation in presence of the nonpolyrnericcrystalline substances forming part of this invention, while in otherexamples a developer is included which has the effect of further fixingthe image to the base firstly by including a substance in the liquiddeveloper which will fix the developed image to the base and sec ondlyby first using a developer without fixing properties and then efiectinga subsequent treatment to fix the image to the base. It will be realisedthat this invention is not restricted by the materials and combinationsthereof cited in the following examples as one skilled in the art canutilise the teachings of this invention to select and to combine otherorganic nonpolymeric crystalline substances for the production ofelectrophotographic and electrostatic coatings.

EXAMPLE 1 A coating composition was prepared as follows:

Grams Phthalic anhydride (nonpolymeric substance) 50 was dissolved inEthyl alcohol 200 Acetone 200 and to this solution was addedPhotoconductive zinc oxide 200 The mixture was ball-milled for 4 hoursand then applied as a coating to paper. The coated paper was air driedand dark adapted for 24 hours after which it was used to produce anelectrophotogra phic print thereon in the following manner:

The coated paper was placed on a metallic base plate in absence ofactivating radiation and subjected to a corona discharge from a seriesof points held at a potential of 7.5 kv. negative in relation to thepositive base plate whereby the coated surface facing the said pointsaccepted a negative surface charge of volts. A light pattern was thenprojected on to the charged surface and the resulting latentelectrostatic image -was then rendered visible by developing, applyingthe so-called dry-method on one sample and the so-called liquid methodon another sample.

In the dry developing method the latent image was developed with acommercially available black toner powder applied from a magnetic brushdeveloping device. The developed image showed high density and nonirnageareas were clean. The image was heat fused.

In the liquid developing method the latent image was developed byimmersing the sheet into a bath containing a liquid developer consistingof a liquid carrier n-heptane having dispersed therein 0.2% by weight atoner paste which paste was prepared by milling the pigmentphthalocyanine blue with bodied linseed oil in equal proportion byweight.

EXAMPLE 2 In the coating composition of Example 1 the quantity of thezinc oxide photoconductor was reduced to 125 grams. Theadhesion-cohesion characteristics of the layer were found to have beenimproved considerably and to be comparable with those of prior art resinbound layers, while the coated surface accepted a surface charge of 200volts.

The coated paper was charged, exposed anddeveloped as described inExample 1.

EXAMPLE 3 Grams Benzoic acid 50 :was dissolved in Ethyl alcohol 350 andto this solution was added Photoconductive zinc oxide 150 The mixturewas ball-milled and applied as in Example 1, and was found to be usableas a hotoconductive layer when used in conjunction with the dry toningtechnique of Example 1.

The above mixture was ball-milled and applied as a coating to paper asin Example 1.

The thus coated paper was charged, exposed and developed as in Example 1except that the liquid developer contained 5% by weight of hydrogenatedrosin dissolved in the carrier n-heptane to apply simultaneously withdeveloping an overall protective layer to the surface.

EXAMPLE 5 The salicylic acid of Example 4 was replaced by thenonpolymeric substance sulphosalicylic acid.

EXAMPLE 6 The benzoic acid of Example 3 was replaced by 2- furoic acidas the nonpolymeric substance.

The coated paper was suitable for use with each of the dry and liquiddeveloping processes described in Example 1.

EXAMPLE 7 The 2-furoic acid of Example 6 was replaced by an equal weightof the nonpolymeric substance maleic anhydride. The surface containingthe developed image was dipped in a 7% solution of the resin ester gumin petroleum ether for protective purposes and dried.

6 EXAMPLE 8 The zinc oxide of Example 1 was replaced by thephotoconductor mercury iodide.

EXAMPLE 9 The zinc oxide of Example 2 was replaced by the photoconductortetragonal lead monoxide.

EXAMPLE 10 The zinc oxide of Example 4 was replaced by thephotocon-ductor cadmium selenide.

EXAlMPLE 11 The zinc oxide of Example 6 was replaced by thephotoconductor cadmium selenide.

The above mixture was ball-milled and applied as a coating to paper asin Example 1.

The coated paper was charged and exposed as in Example 1 but it wassuitable only for the dry developing process as the myristic acid wasnot resistant to the solvent n-heptane contained in the liquiddeveloper. The surface containing the developed and heat fused image wassprayed with an acrylic type protective medium.

EXAMPLE 13 The tetragonal lead monoxide of Example 12 was replaced bythe photoconductor mercury iodide.

EXAMPLE 14 The 'myristic acid of Example 12 was replaced by thenonpolymeric substance lauric acid.

EXAMPLE 15 A paper coated with the composition of Example 4 was chargedand exposed to X-rays. The object radiographed comprised an aluminumalloy coating with maximum wall thickness of one centimeter. The tubewas operated at kvp.

A continuous tone radiograph was obtained by developing the exposedsheet in the liquid developer of Example 1.

EXAMPLE 16 A paper coated with the dye sensitised composition of Example36 was charged and exposed to X-rays using a salt-screen placed againstthe charged surface. The saltscreen was one manufactured by DuPont andbranded HiSpeed. The object radiographed and the tube voltage were as inExample 17.

A continuous tone radiograph was obtained by developing the exposedsheet in the liquid developer of Example 1.

EXAMPLE 17 Grams Phenolphthalein 50 was dissolved in Ethyl alcohol 225Acetone 225 and to the solution was added Photoconductive zinc oxide 200The mixture was ball-milled and coated as in Example 1, and was found tobe suitable for use with each of the dry and liquid toning techniquesdescribed in Example 1.

7 EXAMPLE 1s The photoconductive zinc oxide of Example 17 was reduced inquantity to 150 grams. The so-formed coating showed improvedadhesion-cohesion characteristics when compared with that of Example 17,while the images developed on this coating were also free of backgroundnonirnagewise toner deposition.

EXAMPLE 19 Grams Ortho-phenyl phenol 70 was dissolved in Ethyl alcohol420 and to the solution was added Photoconductive zinc oxide 175 Themixture was milled and the coating used as in Example 1.

prepared and EXAMPLE 20 Grams Para-phenyl phenol 80 was dissolved inEthyl alcohol 400 and to the solution was added Photoconcluctive zincoxide 160 The mixture was milled and the coatingprepared and used as inExample 1.

EXAMPLE 21 The zinc oxide of Example 17 was replaced by 200 grams of thephotoconductor mercury iodide.

EXAMPLE 22 The zinc oxide of Example 17 was replaced by 250 grams of thephotoconductor tetragonal lead monoxide.

EXAMPLE 23 The phenolphthalein of Example 18 was replaced with an equalweight of phenyl salicylate.

EXAMPLE 24 Grams Para-amino benzine 35 was dissolved in Ethyl alcohol210 and to the solution was added Photoconductive zinc oxide 87 Themixture was milled and the coating prepared and used as in Example 1.

EXAMPLE 25 The para-amino benzine of Example 24 was replaced with anequal weight of n-tert-butyl acrylamide.

EXAMPLE 26 The para-aminobenzine of Example 24 was replaced with anequal weight of acetophenetidin.

EXAMPLE 27 Grams Beta-naphthyl-methyl ether 70 was dissolved in Ethylalcohol 350 Ethyl ether 70 and to the solution was added Zinc oxide 175The coating was prepared and used as in Example 1.

EXAMPLE 28 The beta-naphthyl-methyl ether of Example 27 was replacedwith an equal weight of dimethyl terephthalate.

EXAMPLE 29 The beta-naphthyl-methyl ether of Example 27 was replacedwith an equal weight of l-naphthyl salicylate.

The diphenyl guanidine of Example 30 was replaced with an equal weightof para-benzyl aniline.

EXAMPLE 3 2 The diphenyl guanidine of Example 30 was replaced with anequal weight of 2,4,6-trichloroaniline.

EXAMPLE 33 Grams Phenolphthalein 25 Phthalic anhydride 25 were dissolvedin Ethyl alcohol 225 Acetone 225 and to the solution was addedPhotoconcluctive zinc oxide 200 The coating was prepared and used as inExample 1. Adhesion-cohesion characteristics of this coating werecomparable with those of prior art resin bound photoconductor layers.

EXAMPLE 34 Grams Phenolphthalein 20 Phthalic anhydride 40 were dissolvedin Ethyl alcohol 200 Acetone 200 and to the solution was added Zincoxide 180 The coating was prepared and used as in Example 1.Adhesion-cohesion characteristics of this coating were also comparablewith those of prior art resin bound photoconductor layers.

EXAMPLE 35 Grams Phthalic anhydride 50 was dissolved in Ethyl alcohol225 Acetone 125 and to the solution was added Photoconductive zinc oxide116 Anatase titanium dioxide 9 The coating was prepared and used as inExample 1.

This coating was also found to possess excellent adhesion-cohesioncharacteristics, and to hold a surface charge of 275 volts.

EXAMPLE 36 Grams Phthalic anhydride was dissolved in Ethyl alcohol 360Acetone 200 and to the solution was added Photoconcluctive zinc oxide200 which had been coated with a dye solution containing Rose bengal0.05 Brilliant green 0.12 Dibromofluorescein 0.04

Distilled water and dried prior to mixing with the phthalic anhydridesolution.

The coating was prepared and used as in Example 1,

and was found to be of a photographic speed of response 8 times fasterthan the nondye-sensitized coating of the same general formulation asdescribed in Example 2.

EXAMPLE 37 The phthalic anhydride of Example 36 was replaced with 67grams of phenolphtha'lein.

EXAMPLE 3 8 The phthalic anhydride of Example 36 was replaced with anequal weight of salicylic acid.

EXAMPLES 39-41 The dye solution of Examples 36-38 was replaced withGrams Erythrosin B 0.08

Ethyl alcohol 150 EXAMPLES 42-44 The dye sensitised zinc oxide ofExamples 36-38 was replaced with an equal weight of high speedpanchromatic zinc oxide of the type disclosed in United States PatentNo. 2,727,807. A further enhancement in the speed of response wasobtained.

EXAMPLE 45 The phenolphthalein of Example 17 was reduced to 25 grams.Adhesion-cohesion characteristics of the so-formed photoconductor layerwere found to be adequate for electrophotographic processing.

EXAMPLE 46 The phthalic anhydride of Example 2 was increased in quantityto 125 grams and the weight of solvent was doubled.

The so-forrned coating also possessed excellent adhesion-cohesioncharacteristics while the increase in organic nonpolymeric crystallinesubstance content did not cause nonimage charge retention and tonerdeposition in background areas, as would normally occur with a prior artresin bound photoconductor layer containing equal weight ofphotoconductor and resin binder.

What we claim is:

1. An electrophotograp-hic process comprising the steps of exposing anelectrostatically charged photoconductive insulating layer on a sheetselected from the group consisting of paper and metal to a light patternto form an electrostatic image on said layer and developing the image onsaid layer by applying electroscopic particulate material, saidphotoconductive insulating layer consisting essentially of particles ofan inorganic photoconductor and an organic nonpolymeric substance ofcrystalline structure which is not a film-former and does not form abinder like matrix containing the photoconductive particles embeddedtherein and which substance thereby permits sedimentation of thephotoconductive particles in closely packed form without inhibiting thecharge holding and photoresponse characteristics of the photoconductorand forming with the photoconductor a coating of sufiicient mechanicalstrength to withstand described process, said organic substance beingselected from the group comprising organic acids, organic anhydrides,amine derivatives, phenol derivatives, esters, and aniline derivatives,and being nonreactive with the photoconductor at ambient temperatures.

2. An electrophotographic process according to claim 1 wherein theinorganic particulate photoconductor is present in the proportions from1 to 8 parts by weight to 1 part by weight of the organic nonpolymericsubstance of crystalline structure.

3. An electrophotographic process according to preceding claims 1 and 2wherein at least one sensitising dye is incorporated within thephotoconductive insulating layer.

4. An electrophotographic process according to claim 1 wherein thevolume resistivity of the organic nonpolymeric substance of crystallinestructure is at least 10 ohm cm.

5. Electrophotographic printing element comprising a support selectedfrom the group consisting of paper and metal and coated thereon aphotoconductive insulating layer consisting essentially of particles ofan inorganic photoconductor and an organic nonpolymeric substance ofcrystalline structure which is not a film-former and does not form abinder like matrix containing the photoconductive particles embeddedtherein and which substance thereby permits sedimentation of thephotoconductive particles in closely packed form without inhibiting thecharge holding and photoresponse characteristics of the photoconductorand forming with the photoconductor a coating of sufficient mechanicalstrength to withstand described process, said organic substance beingselected from the group comprising organic acids, organic anhydrides,amine derivatives, phenol derivatives, esters, and aniline derivatives,and being nonreactive with the photoconductor at ambient temperatures.

6. Electrophotographic printing element according to claim 13 whereinthe inorganic particulate photoconductor is present in the proportionsfrom 1 to 8 parts by weight to 1 part by Weight of the organicnonpolymeric substance of crystalline structure.

7. Electrophotographic printing element according to preceding claims 13and 14 wherein at least one sensitising dye is incorporated within thephotoconductive insulating layer.

8. Electrophotographic printing element according to claim 13 whereinthe volume resistivity of the organic nonpolymeric substance ofcrystalline structure is at least 10 ohm cm.

References Cited NORMAN G. TORCHIN, Primary Examiner.

JACK C. COOPER, Assistant Examiner.

